If relations are admitted, pluralism may state that things relate only externally or only internally, or dropping the necessary condition some combination of both. Monism might state that there are no discreta at all, but only "modulations" of a ONE whose internal relation is simply the ONE to itself, and thus external relations are not. On the other hand, I do not think monism could logically opt for external relations only, as if there is only ONE then that self-same identity would have no other to relate to (some "other" to stand in as the exterior of the pair).
For a radical pluralism that denies internal relations the main problem seems to be that self-same "essential" identity suffers (where each thing is so internally related so as to be "it" and not some other). If a thing relates to itself so as to be that same thing through time then the internal relation must hold at some minimum. If external relations are denied on the other hand, then the problem is quite different. Communication between any two distinct entities begs a form of relation that cedes into the internal from an external point (which would mean that there could not be "only" external relations for the radical pluralist). If the communication is taken up into a new third item, then we must question in what sense the first two items were what they were as self-same identities without begging infinite regress or without begging the supposed absolute nature of partitioned individualities. With the abolition of external relations, then, we cannot account for any real communication between any two things, let alone for how any two distinct items relate their self-same identity to each other.
Several qualifications are in demand here. a.) there is a difference between intrinsic relations and intrinsic properties or dispositions, b.) there may be properties or dispositions which are themselves fully relational both internal and external (only being a matter of relation to the self, only relating to others, or to both self and others). For a detailed break-down of one's options, G.E. Moore's famous paper "External and Internal Relations" (1919) is a good place to start, or this paper: "All Relations are Internal."
Hartshorne defends asymmetrical relations and temporalizes external relations. He ascribes "the past" to all internal relations. In my judgment, this solves many of the problems presented above.
All external relations are to "contemporaries and the future." To be a discrete entity is to possess internal relations for the requirement of self-same identity (as possessing that specific past for that specific individual). On the other hand, that entity, like all others, relates to its future, as a matter of external relation. Internally, however, as an intrinsic feature of that thing, it must be able to relate to that specific future. In this way there is one form of relation that "exhausts" discreta (even in some form of negation): temporality. Temporality and change "exhaust" the notion of what any discrete item "can be" in the sense that it is necessarily a part of its self-same identity as a condition for that identity. For those discreta which are "eternal" temporality must be negated so that the description "eternal" meaningfully applies.
Note: In a related personal correspondence, good friend Jason Hills of immanent transcedence blog relates this temporality to a starting point for transcendence (what in my Peirce book I've referred to as "lateral transcedence"), where Jason (rightly, in my opinion) states that he attempts to think this becoming "laterally" or "sideways." This is deeply Heideggerean and it has dawned on me that Jason and I both included Heidegger in our dissertations alongside American figures taking up at least in part the theme of temporality and transcendence (he dealt with Dewey, I dealt with Peirce). Jason also made the interesting remark that internal relations - if all relations are internal save for change or temporal becoming ,which would be contemporary and future, make for immanence. Then the external relation would be the beginning point for transcendence as a future-oriented form of relation, one that is fully natural and "along side" or on the same plane as the internal and immanent.